1. Field of the Invention
The present invention generally relates to methods for preparing quaternary ammonium salts and low hazard corrosion inhibitors using those salts. More specifically, the present invention is directed to methods particularly useful for producing pyridinium and quinolinium salts, to low hazard corrosion inhibitors using those salts and to aqueous cleaning solutions using those inhibitors.
2. Description of the Background
Scale comprised of insoluble salts is typically found on the surface of all types of metal equipment in which water is evaporated or heat transfer occurs. These salt deposits are particularly undesirable because of their negative impact on the heat transfer efficiency of the equipment. Because the equipment loses heat transfer efficiency as these deposits build up, it is necessary to periodically clean the equipment to remove the deposits. Industrial cleaning service companies often provide the required cleaning services for this equipment, e.g., the boilers and heat transfer equipment of utilities and industrial plants.
The undesirable scales which must be removed generally comprise calcium and magnesium salts deposited during the evaporation of hard water. Exemplary of these scales are deposits including calcium carbonate, calcium sulfate, calcium phosphate and calcium oxylate. While calcium and magnesium salts comprise the majority of these deposits, salts of other materials may be encountered. Scales high in iron content, e.g., scales including magnetite or hematite, often must be cleaned.
The conventional cleaning operations rely upon the circulation of aqueous cleaning solutions through the equipment, e.g., boilers, heat exchangers and associated piping in an effort to dissolve the salt deposits comprising the scale. Often, these cleaning solutions are heated to temperatures above the boiling point of water. In many early cleaning efforts highly acidic solutions were circulated through the equipment to dissolve the calcium and magnesium salts found in the hard water scale and the magnetite and hematite deposits encountered in high iron scales. As cleaning operations became more sophisticated, solutions containing agents capable of complexing the metals associated with the deposited salts were circulated in order to loosen and dissolve the scale. Ammonia has been used as an alkaline complexing agent for this purpose. See, e.g., the disclosure in U.S. Pat. No. 3,413,160. More recent developments have included complexing agents based upon ethylenediaminetetraacetic acid and related compounds.
Because many of these cleaning solutions are, themselves, corrosive to the metal components of the equipment being cleaned, the solutions must include appropriate corrosion inhibitors. For example, aliphatic pyridinium and quinolinium salts, together with sulphur-containing compounds, have been employed successfully as corrosion inhibitors in these solutions. See, e.g., the disclosure in U.S. Pat. No. 4,637,899 which is incorporated herein by reference. While these corrosion inhibitors provide the desired protection of the metal surfaces, they are often contaminated with unsafe and/or toxic byproducts, e.g., solvents and unreacted. reactants, resulting from the methods by which they were prepared. Accordingly, these corrosion inhibitors and cleaning solutions produced therefrom can present dangers to the employees working with them. Another danger associated with the use of these corrosion inhibitors is the low flash point, often less than 100xc2x0 F., resulting from the solvents and resistants which were used in their manufacture. Still another danger may result from the toxicity of its sulfur-containing compounds employed in these corrosion inhibitors. For example, ethylene glycol monobutyl ether is a toxic chemical used as a solvent and carried along with the aliphatic pyridinium salts used in the methods and corrosion inhibitors disclosed in the ""899 patent. Thiourea is an undesirable sulfur-containing compound typically used in these inhibitors and methods.
As environmental and worker safety concerns have increased, the need to employ less toxic corrosion inhibitors and cleaning solutions has increased. Further, as OSHA requirements and worker safety issues have evolved, the benefits of employing corrosion inhibitors and cleaning solutions with higher flash points has become clear. The industrial cleaning service industry has continued to seek improved corrosion inhibitors and cleaning solutions for use in commercial, scale cleaning operations. The known methods and solutions have not solved these problems. Thus, there has been a long felt but unfulfilled need in the industrial cleaning service industry for less toxic and safer corrosion inhibitors and cleaning solutions. The present invention solves those needs.
The present invention is directed to methods for preparing quaternary ammonium salts and particularly pyridinium and quinolinium salts. These methods are particularly useful in preparing low hazard corrosion inhibitors characterized by low toxicity and high flash points. These low hazard corrosion inhibitors are particularly useful in aqueous cleaning solutions for safely and effectively removing scale deposits from the interior of boilers and heat exchangers.
In the methods of the present invention a quaternary ammonium salt is prepared by contacting a tertiary ammonium compound with a second compound having the formula RX where R is aliphatic, substituted aliphatic or alkyl aryl and X is an anion. Preferably R is selected from the group consisting of alkyl and alkyl aryl moieties having from about 6 to about 18 carbon atoms and X is a halide, most preferably chloride or bromide. The foregoing compounds are reacted in a solvent selected from the group consisting of propylene glycols, propylene glycol ethers and mixtures thereof, most preferably dipropylene glycol methyl ether. The reaction is conducted at a temperature greater than about 65xc2x0 C., preferably in the range of about 75xc2x0 C. to about 125xc2x0 C. Optionally, the reaction may proceed in the presence of water. In its most preferred embodiment, the present invention comprises a method for preparing dodecyl pyridinium bromide by contacting pyridine with dodecyl bromide in dipropylene glycol methyl ether at a temperature above about 65xc2x0 C.
The foregoing methods of the present invention produce mixtures of reaction product and solvent characterized by lower toxicity than mixtures prepared by conventional methods for preparing similar quaternary ammonium salts. Accordingly, salts prepared in accord with the present invention may be used to prepare improved, low hazard corrosion inhibitors. Not only do such corrosion inhibitors exhibit lower toxicity, but they also are characterized by higher flash points than for similar inhibitor compositions prepared by prior methods. These improved corrosion inhibitors may be used to prepare safer aqueous cleaning solutions.
A low hazard composition useful for inhibiting corrosion of steel contacted by organic acids, chelating agents or sulfuric acid may comprise the reaction product of the foregoing methods, a sulfur-containing compound and a nonionic surfactant. These corrosion-inhibiting compositions may be prepared in the solvent in which the reaction product was prepared, in water or in a mixture of both. Particularly useful corrosion inhibitors may comprise 20-50 percent-by-weight of a mixture of the quaternary ammonium compound prepared by the foregoing methods of the present invention and the solvent used in preparing that compound, about 1-10 percent-by-weight of a sulfur-containing compound and about 0-10 percent-by-weight of a nonionic surfactant. The balance of the corrosion inhibitor composition may comprise water, the solvent or mixtures thereof. Not only are these corrosion inhibitors less toxic, but they are safer to handle, typically having flash points at least about 140xc2x0 F. Such corrosion inhibitors are characterized by both lower toxicity and higher flash point than similar compositions prepared by prior methods. These improved qualities may be attributed to the use of propylene glycol and propylene glycol ether as reaction solvents.
In a final aspect of the present invention, improved aqueous cleaning solutions employing the quaternary ammonium salts produced by the foregoing methods are disclosed. Typical of these solutions are aqueous cleaning solutions having a pH from about 1-10 and comprising at least one organic, acid selected from the group consisting of alkylene polyamine polyacetic acids, hydroxyacetic acids, citric acid and mixtures or salts thereof, together with an effective amount of a corrosion inhibitor including quaternary ammonium salt prepared in accord with the methods of the present invention. In another aspect of the present invention, aqueous cleaning solutions comprising at least one acid selected from the group consisting of sulfuric acid, hydrochloric acid and phosphoric acid, together with an effective amount of a corrosion inhibitor including a quaternary ammonium salt produced by the methods of the present invention, are disclosed. The cleaning solutions of the present invention are safer than those produced using prior methods.
The present invention provides methods for producing quaternary ammonium salts useful in applications where reduced hazard levels, e.g., low toxicity, are required. The present invention provides methods for producing improved corrosion inhibitors characterized by lower toxicity and higher flash points. Accordingly, these corrosion inhibitors provide health and safety benefits to the industrial cleaning industry where they may be used to formulate industrial cleaning solutions.
Thus, the long felt but unfulfilled need in the industrial cleaning industry for safer corrosion inhibitors and aqueous cleaning solutions has been met. These and other meritorious features and advantages of the present invention will be more fully appreciated from the following detailed description and claims.